How do I make a high poly model from a low poly model and a normal map?
Moonboots was kind enough to provide one link in the comments, which I'll check out further.  Scroll down that Reddit page to find usage instructions; it works for me.
Another option here is njob ( http://charles.hollemeersch.net/njob/ ).
Njob just outputs 8bpc height maps.  Neither NormalToHeight nor Njob can properly handle texture tiling.
Note that conversion of normal maps to displacement maps is inexact.  There exists some data necessary for displacement that isn't present in a normal map.  Neither of those utilities take your UV tangents into account, and you'll find that displacement is incorrect as your UV rotates.  You will need to adjust displacement parameters to eye.
It's also possible for a normal map to encode for impossible angles, that can't be represented by geometry.
However, I know that it is possible to get reasonable results using Njob.  I sometimes convert from normal to bump in order to not have to worry about UV tangents during material creation, or to smooth out a rendered displacement-- smooth shaded normals interpolate differently than position, so rendering as normals and converting to height can create a smoother displacement map than just rendering height/position.